1. Field of the Invention
The present invention relates to fiber-optic communications networks, and more particularly, to optical network equipment such as optical amplifiers in which pump powers are controlled to permit pumping under a wide dynamic power range and having stable operation at low pump powers.
2. Description of the Related Art
Optical amplifiers, such as the well-known Erbium Doped amplifier, are pumped by a laser source, typically a laser diode, to derive signal gain. Conventional optical amplifier systems implement power control of the amplifier by changing the DC drive current applied to the pump laser diode so as to control the pump power supplied to a gain medium. This method is used to ensure that the optical amplifier operates at a certain power set point. Unfortunately, when the set point is such that the pump laser is operated near its lasing threshold, reflections in the optical path (from anywhere between the pump and the gain medium) can cause drastic fluctuations in the pump operation. When the pump laser is operated close to its threshold, such reflections and other perturbing effects may cause the pump laser to either drop below threshold or to change its output mode. Such fluctuations cause undesirable wide excursions in the pump output power that limit the stability in both the pump and the amplifier under low power operation. Additionally, pump bistability may occur at currents significantly above threshold, once again limiting the stability and operation of the amplifier/pump.
Conventional solutions to stabilize the output power of the amplifier pump lasers include the use of Fiber Bragg Gratings (FBGs) to stabilize pump wavelength and/or power and pump current dither circuits to stabilize pump power. Pump stabilization designs using FBGs alone impose tight pump laser manufacturing specifications on parameters such as front facet reflectivity and pump laser wavelength control in order to control the detuning between the FBG wavelength and the laser wavelength. The FBG designs for good power stability also typically require long fiber lengths and one or more FBGs in the fiber pigtail at one or more meters from the pump chip, both leading to higher cost.
Although the use of FBGs provides a stable wavelength of operation for the pump at high output powers, it, unfortunately, does not ensure pump power stability at very low power. As the drive current to an externally stabilized pump laser is increased through and above the lasing threshold, the device will commence laser operation in a single mode external cavity state and will, later, transition to the more stable multi-mode coherence collapse regime. The critical current at which the stable multi-mode operation occurs is determined by many interacting pump parameters.
As previously mentioned, optical amplifiers are pumped by a laser diode to derive signal gain. Generally, the drive current to the pump laser diode is controlled in a proportional manner such that an increase in signal power is obtained by an increase in drive current. However, at low power, pump power stability is compromised, as operation at currents near threshold causes large relative power fluctuations. Additionally, at high power, kinks and other non-linear characteristics of the L-I curve limit maximum operating power. Therefore, the overall dynamic range of amplifiers is typically limited to the linear portion of the L-I curve.
As the foregoing illustrates, there is a need in the art for an improved system and method for controlling optical amplifier output power.